Architecture: This tent is worth making a song and dance about: Llangollen's new pavilion for the annual eisteddfod is a model of cheap, innovative theatre design, says Jonathan Glancey

THE annual Llangollen International Eisteddfod in north Wales must be one of the most assertively 'traditional' events of the music year. This is the place to hear massed Welsh choirs and this is where, last week, singers and dancers from 47 countries came to perform traditional songs and dances and 150,000 people came to see and hear them. This is where the young Geraint Evans, Tito Gobbi and Luciano Pavarotti cut their operatic teeth.

Yet this year's eisteddfod took place under the vast tented roof of an unapologetically modern structure, the roots of which date back in style no earlier than the Montreal Expo of 1967, when the German pavilion was the first public building to flaunt a stretched membrane roof.

The Royal International Pavilion in Llangollen, opened by the Queen last week, is exactly the building the festival had been waiting for: cheap, flexible, durable and festive.

From a distance the armadillo-like tented roofs look like some creature from outer space foraging in the Vale of Llangollen, or perhaps some outsize Bedouin tent. Yet although unfamiliar, its hump-backed shape has been designed to echo the surrounding hills. While the fabric roofs look ethereal in the sun, the stone walls appear to be buried; in fact, earth has been piled up around the walls to give the feeling that the building, although modern, is rooted in the landscape.

The pavilion is really a semi-permanent tent - a pair of hi-tech PVC membranes stretched over a 70ft-high steel arch and supported by walls made of reconstituted stone into which are set offices, changing rooms and a cafeteria. The structure has been constructed by the engineering company Atelier One so that a very small team of people, without any equipment, can alter its layout during the year. The idea is that during the 12 hectic days of the eisteddfod, it can be extended to a maximum capacity of 6,000 while during the rest of the year it can be shrunk back and used as a 1,800- seat opera house or theatre or, in its most condensed form during the wettest winter months, as a sports hall and community and conference centre.

This type of tented structure, popularised by Michael Hopkins's 1988 design for the Mound Stand at Lord's Cricket Ground in London, is both flexible and relatively cheap. The Llangollen pavilion cost pounds 3m, which is very low for a well equipped 6,000-seat theatre. The Eisteddfod Committee, although assisted by grants from Glyndwr District Council, Clwyd County Council, the Welsh Tourist Board and the European Community, had little money to spend on the pavilion, but had

promised itself, its performers and its patrons that it would open this summer.

Originally, the committee was to have organised an international competition for the design, but it decided that this would take too long. Instead, it appointed D Y Davies Associates, a practice known for its hard-nosed approach to commercial design (office blocks, business parks and hotels). David Davies, the Wrexham-born chairman of D Y Davies, had already won a commission in 1985 for his company to design a 500-seat pack-away theatre for the eisteddfod (designed by his partner John Muir) and had been asked to judge the proposed international design competition for the new pavilion.

'The building is proof,' says Mr Davies, 'that commercial architects - as we are inevitably labelled - are just as capable of designing an adventurous and festive building as more avant- garde architects. As we're so used to working to tight budgets, that the idea of designing and building a large and highly serviceable theatre - about the same volume as the Albert Hall - for just pounds 3m was not one that we thought particularly difficult.'

While the stone-clad and slate- roofed core of the building are designed for a long life, the PVC fabrics are guaranteed for 15 years. 'In practice they will probably last much longer,' says Vernon Almeida, of D Y Davies. 'They are relatively inexpensive so when they finally have to be replaced, the process wil be cheaper and easier than rebuilding a roof. The fabrics are maintenance free and they should stay very clean; the Vale of Llangollen is not a grimy area and, anyway, it's always raining, so any dirt is soon washed away.'

The Llangollen pavilion is a form of architecture, both modern and traditional, matter-of-fact and festive, that is setting the style for a large number of sports stadiums and cultural centres throughout Europe - such as the Olympic pavilions in Barcelona and Parc de la Villette, the popular science park in Paris. It is no less appropriate for such traditional British institutions as the eisteddfod and the MCC: an architecture worth making a song and dance about.